How to get a new array from a specific array

Question

How do I get a new array from a specific array?

I don't known how to handle an Array like this:

let arr1 = [
  {name: 'a', age: 1},
  {name: 'a', age: 2},
  {name: 'a', age: 3},
  {name: 'b', age: 1},
  {name: 'b', age: 2}
  {name: 'b', age: 3}
  {name: 'c', age: 1}
  {name: 'c', age: 2}
  ...
]

"I want this result: from each item with the same name, pick biggest age value"

let arr2 = [
  {name: 'a', age: 3},
  {name: 'b', age: 3},
  {name: 'c', age: 2} 
  ...
]

`const arr2 = arr1.filter( a => a.age === Math.max( ...arr1.filter( b => b.name === a.name ).map( ({ age }) => age ) ) );` — Paul, Jul 01 '19 at 23:47
I think that's what you want, but I'm on mobile so I can't post an answer with code easily. — Paul, Jul 01 '19 at 23:48

Jack Bashford · Answer 1 · 2019-07-02T01:14:11.067

3

Just use filter to select all items with the highest age for their name.

let arr1 = [{name:'a',age:1},{name:'a',age:2},{name:'a',age:3},{name:'b',age:1},{name:'b',age:2},{name:'b',age:3}];

const biggest = arr1.reduce((a, { name, age }) => {
  a[name] = a[name] || age;
  age > a[name] ? a[name] = age : a[name];
  return a;
}, {});

let arr2 = arr1.filter(({ name, age }) => biggest[name] == age);

console.log(arr2);

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edited Jul 02 '19 at 01:14

answered Jul 01 '19 at 23:42

Jack Bashford

43,180
11
50
79

Much cleaner than mine... +1 :-D – Ben Carey Jul 01 '19 at 23:43
1

Nice trick with the console snippet btw, I wasn't aware you could do that! – Ben Carey Jul 01 '19 at 23:45
Yeah - that just lets the console output become bigger if it's required – Jack Bashford Jul 01 '19 at 23:47
That doesn't answer his question. He wants the biggest number. It isn't necessarily 3. – Billy Hudson Jul 02 '19 at 00:26
Biggest number - my bad, thought it was three. Fixed @BillyHudson. – Jack Bashford Jul 02 '19 at 00:30
1

Better @BillyHudson? – Jack Bashford Jul 02 '19 at 00:30
@JackBashford That snippet doesn't seem to work if I change the input array to something like `a:1, a:2, a:3, b:1, b:2, b:4`, etc. – Billy Hudson Jul 02 '19 at 00:35
Hmm - oh I see, so the highest of *each `name`*. That's better - I'll fix it. – Jack Bashford Jul 02 '19 at 00:40
Okay fixed. Could those who downvoted please remove their votes, as the answer now works. – Jack Bashford Jul 02 '19 at 00:42

score 2 · Accepted Answer · answered Jul 02 '19 at 00:09

You can use a combination of reduce and Object.values

 const arr1 = [
  {name: 'a', age: 1},
  {name: 'a', age: 2},
  {name: 'a', age: 3},
  {name: 'b', age: 1},
  {name: 'b', age: 2},
  {name: 'b', age: 3},
  {name: 'c', age: 1},
  {name: 'c', age: 2},
]

const maxNames = arr1.reduce((maxNamesObject, item) => {
  const { name, age } = item
  const maxItem = maxNamesObject[name]
  if (!maxItem || age > maxItem.age) {
    maxNamesObject[name] = item
  }
  return maxNamesObject
}, {})

const output = Object.values(maxNames)

console.log(output)

score 0 · Answer 3 · answered Jul 01 '19 at 23:41

0

Extracted from this answer: How to filter object array based on attributes?

You can use the Array.prototype.filter method:

let arr2 = arr1.filter(function (obj) {
  return obj.age === 3;
});

answered Jul 01 '19 at 23:41

Ben Carey

16,540
19
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sorry! this not my want, arr1 just a stucture like this, I have edit my new question! – EVDS Jul 01 '19 at 23:44
@EVDS - you need to expand further in your question. Both posted answers will filter the array and return a new array based on the specified filter – Ben Carey Jul 01 '19 at 23:47
@BenCarey what happens to age === 2 || age === 1 as the higest of it key?(name) – Wale Jul 02 '19 at 00:07
@BenCarey Sorry about that. – Wale Jul 02 '19 at 00:10

score 0 · Answer 4 · answered Jul 02 '19 at 00:46

This is a more beautiful solution:

let arr1 = [
  {name: 'a', age: 1},
  {name: 'a', age: 2},
  {name: 'a', age: 3},
  {name: 'b', age: 1},
  {name: 'b', age: 2},
  {name: 'b', age: 3},
  {name: 'c', age: 1},
  {name: 'c', age: 2},
]

arr2 = Array.from(new Set(arr1.map(e => e.name))).map(e => { 
    return {
        name: e,
        age: Math.max(...arr1.filter(ee => ee.name == e).map(ee => ee.age))
    }
})

console.log(arr2)

Blockquote

score 0 · Answer 5 · answered Jul 02 '19 at 01:49

my solution:

const arr1 = [
  {name: 'a', age: 1},
  {name: 'a', age: 2},
  {name: 'a', age: 3},
  {name: 'b', age: 1},
  {name: 'b', age: 2},
  {name: 'b', age: 3},
  {name: 'c', age: 1},
  {name: 'c', age: 2},
]

let arr2 = arr1.reduce((acc, item) => {      
  let objectExists = acc.find(e => e.name === item.name)
  if (!objectExists) acc.push(item)
  else if (objectExists.age < item.age) {
      acc[acc.indexOf(objectExists)].age = item.age
  }
  return acc
}, [])

console.log(arr2)

Alejandro · Answer 6 · 2019-07-02T02:14:42.673

I didn't notice you update the question. This is not the most pro code.

  let arr1 = [
  {name: 'a', age: 1}, {name: 'a', age: 2}, {name: 'a', age: 3},
  {name: 'b', age: 1}, {name: 'b', age: 2}, {name: 'b', age: 3},
  {name: 'c', age: 1}, {name: 'c', age: 2} ];

  let names = [];
  let output = [];

  for(obj of arr1){
   if (!names.includes(obj.name)) names.push(obj.name);
  }

  for(obj of names){
    let current = arr1.filter(x => x.name === obj)
                      .reduce((a,b) => a.age > b.age ? a : b);

    output.push(current);
  }

    console.log(output);

How to get a new array from a specific array

6 Answers6